Method and apparatus for ascertaining color balance of photographic printing paper

ABSTRACT

The color balance of color photographic printing paper is determined by making three successive exposures of separate portions of a sample sheet of printing paper through a neutral-color optical wedge of linear density gradient, each exposure consisting essentially of a different primary color of light of predetermined intensity and duration to selectively stimulate each of the three color-sensitive emulsion layers in the printing paper. The three images produced upon development provide a means for objective quantitative measurement of the relative responsiveness of each of the emulsion layers using a reflection densitometer to locate points of equal optical density, the relative linear displacement of the points providing color-balance correction factors. An easel is provided with a sliding carriage for positioning the sample sheet of printing paper in proper relationship to the optical wedge, which is fixed in place in an aperture in a light-proof cover. A device for calculating the correction factors from the printed images of the optical wedge is equipped with sliding scale members calibrated for measuring displacement in appropriate units for making color-balance corrections.

BACKGROUND OF THE INVENTION

The present invention pertains to color balancing of photographicprinting paper.

Although the present invention has application in various photographicprocesses, it is particularly useful with processes wherein a positiveprint is produced from a color negative using materials known asintegral tripacks which have three superimposed light-sensitive emulsionlayers. Those skilled in the art will appreciate that it is rarely ifever possible to produce a print with accurate color rendition withoutadjusting for color-balance variations, which are caused by color-mixchanges in lighting conditions and changes in the characteristics offilms and printing papers. Negative-positive processes providesubstantial latitude in making color correction in printing the negativeby selective variation of red, green, and blue light exposures. Thereare two principal techniques for making such exposure variations inpresent use. In tricolor additive printing, three separate exposures ofthe negative to the printing paper are made respectively through red,green, and blue separation filters, wherein the three exposure times arevaried to control color correction. Exposure of the printing paper tovarying amounts of red, green, and blue light, hereinafter referred toas the primary colors of light, is thereby achieved. In white-lightsubtractive printing, a single exposure of the negative to the printingpaper is made through a combination of colored filters, wherein filterdensities and colors are varied to control color correction, therebyselectively filtering out varying amounts of the primary colors oflight. In either case, color control is achieved by selectively varyingthe exposures of the primary colors of light to the printing paper,exposure being basically a product of light intensity and duration. Suchcolored light variations produce corresponding variations in theproduction of complementary colored dyes by the three emulsion layers,red light producing cyan dye, green light producing magenta dye, andblue light producing yellow dye.

Most prior art color balancing techniques employ visual color judgmentsusing test prints. Of particular application to the white-lightsubtractive printing technique is the use of color filters for viewing atest print to make a judgment as to filtering changes for makingsubsequent prints. Less subjective techniques involve the use ofelectronic color analyzers to detect changes in color balance fromnegative to negative. Proper use of such color analyzers requiresinformation as to the color balance of the printing paper being used.Due to changes in storage conditions and age, no two boxes of colorprinting paper have precisely the same color balance. Therefore, uponstarting a new box of printing paper, a test print is made to obtain thenecessary color balance information, which can then be used to reprogramthe color analyzer. Here again even the most sophisticated prior arttechniques resort to subjective visual judgment in obtaining such colorbalance information about the new printing paper to be used. In onepopular technique, a picture is first taken of a gray card, whereuponthe gray card negative is printed through a matrix of colored filtersonto a sample sheet of the printing paper under test. By visualcomparison, a gray patch is then found on the print, enabling theselection of an appropriate filter combination for color balancing thesystem. The color analyzer can then be programmed to such a filtercombination to compensate for the characteristics of the printing paper.Different color negatives can then be tested for color balance by thecolor analyzer, enabling exposure corrections to be made regardless ofwhether tricolor additive or white-light subtractive printing is thenused.

Subjective visual judgment is eliminated in one prior art technique ofascertaining the color balance of photographic printing paper inaccordance with U.S. Pat. No. 3,392,626. While the procedure iseffective, it is also laborious and time consuming. It requiresmeasuring reflection densities corresponding to discrete values of stepwedge density for each of three color exposures, then plotting graphsfrom the density measurements, and finally making numerical calculationsfrom the graphs to provide color balance data.

The present invention provides a greatly simplified objectiveinstrumental technique for obtaining the color balance of photographicprinting paper without reliance on subjective visual judgment andwithout the laborious graphing techniques described in the cited patent.

SUMMARY OF THE INVENTION

In accordance with the present invention, the color balance of colorphotographic printing paper of the type having superimposed emulsionlayers is ascertained by forming separate test images on a sample sheetof printing paper, each image being representative of the responsivenessof one of the emulsion layers, each image being formed using apredetermined linearly varying log exposure, such that changes inoptical density can be measured in terms of changes in displacement,thereby providing objective correction for color balance. A speciallyconstructed easel provided with an optical wedge facilitates exposuresof printing paper under darkroom conditions for forming the test images.A specially constructed calculator facilitates measurement ofdifferential displacement on the test images and determination ofquantitative correction factors.

The novel features believed characteristic of the invention are setforth in the appended claims. The nature of the invention, however, aswell as its essential features and advantages, may be understood morefully upon consideration of illustrative embodiments when read inconjunction with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical elevation of a photographic enlarger for use withthe present invention;

FIG. 2 is a view in perspective of a color analyzer for use with thepresent invention;

FIG. 3 is a view in perspective of a densitometer for use with thepresent invention;

FIG. 4 is a view in perspective of an easel constructed in accordancewith the present invention;

FIG. 5 is a plan view of a sample print made in accordance with thepresent invention; and

FIG. 6 is a view in perspective of a calculator apparatus made inaccordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1, 2 and 3 illustrate known pieces of equipment, which may be usedin practicing the present invention.

FIG. 1 illustrates a general purpose photographic enlarger 10 having ahead 12 which is equipped with a lamp 14 for producing light through alense 16 at selected heights above a baseboard 18. A negative carrier 20is disposed in the head 12 between the lamp 14 and the lense 16. Thehead is provided with a drawer 22 for inserting filters in the lightpath above the negative. In addition, a holder 24 is provided forinserting filters beneath the lense 16. The voltage of the lamp 14 ispreferably controlled by a device 26, the significance of which isdiscussed below.

FIG. 2 illustrates a color analyzer 30, also known as an easel-readingtransmission-type densitometer, a variety of which are presentlymarketed. The color analyzer 30 is equipped with a probe 32 having anaperture 34 through which the light to be analyzed passes. The color ofthe light to be analyzed, whether red, green, blue or neutral, isselected by means of a color-channel control knob 36. A meter 38 havinga needle 40 and density and time scales is provided for controllinglight intensity and exposure time as described below. The color analyzer30 is provided with four attenuator controls 42, 44, 46, and 48, one foreach of the four color channels, which may be used to give quantitativereadings of optical density as determined by their settings.

FIG. 3 illustrates a reflection-type densitometer 50, which is capableof measuring the optical density of a color photographic print. Thedensitometer comprises a probe 52 having a lens 54 through which testlight emerges and reflected light returns. The color of the emergingtest light, whether red, green, blue, or neutral, is selected by meansof a color-channel control knob 56. Reflected light is sensed anddisplayed in units of optical density by a readout 58. A control 60 isprovided for adjusting readings to zero optical density.

FIG. 4 illustrates a specially equipped easel, generally indicated byreference numeral 110, for preparing a specimen of color printing paperfor analysis. The easel 110 has a frame 112 with a carriage 114slideably disposed therein in the manner depicted. Clips 116 areprovided for securing printing paper to the carriage 114. A light-proofcover 118 is hinged to one edge of the frame 112. Fitted into the cover118 is a color-neutral optical wedge 120 of linear density gradient. Ofthe available wedges, it is preferred that the wedge 120 be a precisiontype which varies linearly in log opacity by ±0.50 optical density unitsfrom its midpoint. Also in the cover 118 is an aperture 122 convenientlyspaced from the optical wedge 120. Hinged flaps 124 and 126 are mountedon the cover 118 for selectively covering respectively the optical wedge120 and/or the aperture 122 in the depicted manner. The frame 112 of theeasel 110 is equipped with means for selectively positioning thecarriage 114 in at least three different positions for making separatesuccessive exposures of a color print through the wedge 120. Thepositioning means, which are provided to facilitate proper spatialarrangement of the carriage 114 by the user in complete darkness, maytake the form of mechanical stops 128 providing a resistance to movementthat can be felt as the carriage 114 slides in the frame 112 asindicated by the arrows. The stops 128 may, for example, comprisespring-loaded ball bearings which protrude from the frame 112 as shownto interfere slightly with the leading edge of the carriage 114 as itslideably engages the inside walls of the frame 112. Alternatively, aset of marks (not shown) can be placed at appropriate points on asurface of the frame 112, so that they can be felt by the fingertips forproper positioning of the carriage 114 in the dark.

FIG. 5 illustrates a color print 130 prepared from a representativesample sheet of color printing paper in accordance with the inventivemethod. The print 130 comprises four separate test images. The images132, 134, and 136 are made by a predetermined linearly varying logexposure to a different primary color of light, which may be achievedusing color separation filters and the optical wedge 120 or anyequivalent means, thereby providing a measure of the responsiveness ofeach of the three emulsion layers. The fourth image 138 is made by apredetermined uniform exposure to one of the primary colors of light forpurposes of exposure-time calculation as described below. Because thecarriage 114 of the easel 110 slides in a direction perpendicular to theaxis of varying optical density of the optical wedge 120, the images132, 134, and 136 are perfectly aligned so that points in each of thethree images that correspond to exposure through a common point of theoptical wedge 120 are equidistant from any selected reference line takenperpendicular to the axes of varying optical density of the threeimages.

FIG. 6 illustrates an apparatus, generally indicated by referencenumeral 140, for calculating relative displacement of optical density inthe images 132, 134 and 136 of the print 130. A print holder 142provides a flat surface 144 for mounting the print 130 thereon with theorientation shown. The holder 142 is equipped with a rail 146 havingspring-loaded hinges 148 enabling the print 130 to be clamped along oneedge. The apparatus 140 further comprises two slide carriages 150 and152 of rectilinear shapes which may be positioned over the print 130 onthe holder 142. A flange 154 facilitates sliding movement of the largercarriage 150 along the rail 146 in the direction shown by arrows 156.The vertical mating surfaces of the rail 146 and the carriage 150 shouldbe made as straight as possible so that the interface between the rail146 and the print 130 provides a straight reference line fordisplacement measurements. The larger carriage 150 has sidewalls 158 anda bottom surface 160 permitting sliding movement of the smaller carriage152 therein in the manner depicted by arrows 162. The smaller carriage152 has an aperture 164 adapted to provide a snug fit for the probe 52of the densitometer 50 shown in FIG. 3. The larger carriage 150 has anaperture 166 in the bottom surface 160 permitting light from thedensitometer probe 52 to fall on the print 130. Scales 168 withcalibrated markings are disposed on the smaller carriage 152, andpointers 170 are slideably disposed on the larger carriage 150, enablingthe relative displacement of the two carriages 150 and 152 to bemeasured from a starting point.

The markings of the scales 168 bear an important relationship to thegradient of the optical wedge 120 of FIG. 4. In order to relate changesin optical density of a printed image of the optical wedge 120 tochanges in exposure, the markings are calibrated in units of lineardisplacement reflecting the density gradient of the optical wedge 120.For example, if the gradient of the optical wedge 120 in use is 0.10optical density units per centimeter, then the markings may beconveniently placed at one millimeter intervals to give readings of 0.01optical density units of exposure difference per millimeter ofdisplacement. Those skilled in the art will appreciate that there is adirect relationship between units of differential displacement along theoptically variable axis of the wedge 120 and units of log exposure,thereby permitting changes in displacement to be converted to changes inexposure.

In the embodiment shown, the two sets of scales 168 are calibrated inopposite directions. That is, one scale increases in value from right toleft while the other scale increases in value from left to right. Asingle scale with a center zero point would be a satisfactoryalternative; however, the double scale arrangement provides twice therange of displacement readings in the same space.

A number of suitable alternative embodiments of the calculator apparatus140 will suggest themselves to those skilled in the art. For example,the present invention could as well be practiced by using a similarembodiment wherein the scales 168 are disposed on the larger carriage150, and the pointers 170 are disposed on the smaller carriage 152. Ineither case, the pointers 170 could alternatively be fixed in place andthe scales 168 be made movable. As a further alternative, the largercarriage 150 could be adapted for directly receiving the probe 52 of thedensitometer 50, thus eliminating the need for the smaller carriage 152.In such case, the probe 52 could be provided with a pointer or pointersand the carriage 150 could have a movable scale or scales. Furthermore,a suitably shaped probe 52 could be accommodated to ride along abar-shaped member in place of the carriage 150. Accordingly, theinvention contemplates any system which permits measurement ofdifferential displacements of the probe 52 with respect to points on thepaper 130, enabling the direct calculation of color-balance correctionfactors.

A preferred method of practicing the present invention will now bedescribed by way of an illustrative example. First it is necessary toprepare the enlarger 10 of FIG. 1 to permit the generation of separateexposures in predetermined amounts for each of the three primary colorsof light. The enlarger 10 is set up so that the head 12 is sufficientlyhigh to project onto the baseboard 18 a field of illumination having adiameter roughly three times the length of the optical wedge 120. Thecolor analyzer 30 of FIG. 2 is programmed for sensitivity by setting thered, green, and blue attenuators 42, 44, and 46 either to some commonsetting, or to a reasonable combination of settings that previouspractice has shown will be within striking distance of color balance forthe printing paper under test. Although the settings are relativelyarbitrary, they should not be set so far apart as to exceed the range ofpossible correction as determined by the gradient of the optical wedge120 shown in FIG. 4.

With the color analyzer 30 programmed, the probe 32 is placed on thebaseboard 18 of the enlarger 10. A red filter of good quality opticalgelatin, such as a Kodak Wratten tricolor filter, is placed in theholder 24 beneath the lens 16. Working in the dark so as to confine allmeter readings to the measurement only of light being emitted from theenlarger lens 16, the control 36 on the color analyzer probe 32 isswitched to the red channel. The aperture (not shown) of the enlargerlens 16 is then varied until the meter 38 reads some arbitrarilyselected value, known previously to be suitable, which is defined as thezero point. Next, a suitable green filter replaces the red filter in theholder 24, and the control 36 of the color analyzer 30 is switched tothe green channel. Without altering the aperture setting of the lens 16,the intensity of the light emitted from the lamp 14 is varied until theneedle 40 falls on the previously selected zero point. Although thereare a variety of means for varying the light intensity of the lamp 14,it is presently preferred that the lamp voltage be varied by a devicesuch as the device 26 which is described in U.S. Pat. No. 3,392,626. Thedevice 26, which is serially connected between the lamp 14 and aregulated voltage source (not shown), comprises three channels forselectively operating the lamp 14 at one of three different separatelyadjustable average voltage levels. With the lamp intensities of both thered and green exposures set on attenuators 42 and 44, the device 26 isswitched to its third channel, an appropriate blue filter is inserted inthe holder 24 in place of the green filter, and the control 36 of thecolor analyzer 30 is switched to its blue channel. The voltage of thelamp 14 is then varied by means of the device 26 to once again bring theneedle 40 to the zero point. At this stage, the programmed settings ofthe color analyzer 30 have been translated into voltage settings oncorresponding channels of the device 26, such that subsequent exposuresmay be made at predetermined light intensities corresponding to theprogrammed settings of the color analyzer.

If a device, such as the device 26, is not available for varying thelight intensity of the lamp 14 in the above described manner, variousalternative means can be used to achieve satisfactory results. Forexample, an automatic colorhead enlarger may be used to selectively varythe intensity of the primary colors of light. Available colorheadenlargers enable continuous color variation using dichroic filters.Another satisfactory, though less advantageous, alternative for varyinglight intensity is the use of either color printing or colorcompensating filters in a known manner with a general purpose enlarger.The use of such discrete filters is less desirable because intensityvariations can only be made in discontinuous steps. Whatever means forlight intensity variation is used, however, it is only necessary thatthe predetermined value of light intensity be stable and reproducible inthe darkroom. An additional factor which must be considered in usingoptical filtering means to control light intensity, which does notmanifest itself when using the voltage control means of the citedpatent, is the problem of possible cross-over existing between colorsdue to the inherent limitations of color filters. Such cross-overeffects can be eliminated by repeating the above described procedure forzeroing the color analyzer 30 until it can be switched between primarycolor channels without moving the needle 40 off the zero point.

In addition to determining color balance, the present invention providesa convenient way for determining the overall responsiveness, or printingspeed, of the printing paper being tested, so that an appropriate printdensity may be achieved. Accordingly, a neutral filter of suitableoptical density as determined by prior experience is placed in theholder 24 under the enlarger lens 16. A Kodak Wratten No. 96 neutraldensity filter having an optical density of 0.70 has been foundexperimentally to be satisfactory for this purpose. The overallresponsiveness of the printing paper is keyed to one of the threeemulsion layers, such as for example the red sensitive layer.Accordingly, the previously used red separation filter is placed overthe aperture 34 of the color analyzer probe 32 which remains in positionon the enlarger baseboard 18. The color analyzer control 36 is switchedto its neutral mode. The enlarger light intensity is set at the aboveestablished value for red light using the same intensity control meansas before. That is, if the voltage control means was used, then thecontrol device 26 should be switched to its red channel. Then theneutral channel attenuator 48 of the color analyzer 30 is adjusted untilthe meter 38 indicates an exposure time that has been known by priorexperience to have produced satisfactory results. The setting of theneutral attenuator 48 is recorded for future reference. The neutraldensity filter is removed from the holder 24, and the color analyzerprobe 32 is removed from the enlarger baseboard 18 as preparations forthe next phase of procedural steps begins.

The preferred procedure for making the test print 130 of FIG. 5 will nowbe described. The easel 110 of FIG. 4 is positioned on the baseboard 18of the enlarger 10 shown in FIG. 1, so that the optical wedge 120 willlie in the middle of the projected field of illumination from the lens16. Working now in complete darkness, a sample sheet of color printingpaper is secured in place on the carriage 114 using the clips 116. Thecarriage 114 is then moved to the first of the three stops 128, and thecover 118 is closed to place the optical wedge 120 over a portion of thesample sheet where a first test image will be made. While the flap 124is open to permit exposure through the optical wedge 120, the flap 126is closed to cover the aperture 122. The blue separation filter usedabove in calibrating the intensity for blue light is placed in theholder 24, and the means for controlling the intensity is madeoperational with respect to the predetermined value for blue light. Thatis, if the voltage control means was used, the device 26 is switched toits blue channel; whereas, if filtering was used to control lightintensity, the appropriate blue-light absorbing filter previouslydetermined is placed in the light path. The blue exposure of the printis then made for the predetermined number of seconds.

The second exposure through the optical wedge 120 proceeds in similarfashion. The carriage 114 is moved to the second of the three stops 128,and the green separation filter replaces the blue separation filter inthe holder 24. With the means for controlling the intensity operationalwith respect to the predetermined value for green light, the greenexposure is made for precisely the same time interval as for theprevious blue exposure.

The last of three exposures through the optical wedge 120 is achievedsimilarly using red light. The carriage 114 is moved to the last of thethree stops 128, and the red separation filter is inserted in the holder24 in place of the green separation filter. The means for controllingintensity is made operational with respect to the predetermined valuefor red light, and then the red exposure is made for precisely the sametime as the blue and green exposures.

Finally, a fourth exposure is made for the same duration but without theuse of the optical wedge 120. This is facilitated by the construction ofthe presently preferred easel 110 by simply closing the flap 124 tocover the optical wedge 120 and opening the flap 126 to uncover theaperture 122. The neutral density filter previously used in setting theneutral attenuator 48 of the color analyzer 30 is placed in the enlargerholder 24 in addition to the red separation filter already in placetherein. Without adjusting the intensity control means, a final exposureof the printing paper is made for the same time duration as before.

Upon development of the sample sheet of printing paper using the usuallaboratory processing techniques, four test images will appear asdepicted in FIG. 5. A yellow image 132, a magenta image 134, and a cyanimage 136, all of varying optical density, represent respectively theblue, green, and red exposures through the optical wedge 120. A uniformdensity cyan image 138 represents the red exposure made through theaperture 122.

The procedural steps involved in determining the responsiveness of thelight-sensitive emulsion layers in the sample print 130 will now bedescribed with particular reference to FIG. 6. The sample print 130 ispositioned on the holder 142 using the rail 146 to clamp one edge of theprint 130 against the mounting surface 144 with the parallel axes ofimages 132, 134, and 136 oriented perpendicular to the facing verticaledge of the rail 146. Slide carriage 150 is positioned over the print130 and mounting surface 144 with its flange 154 riding on the rail 146.The probe 52 of the reflection densitometer 50 shown in FIG. 3 is fittedinto the aperture 164 of the smaller slide carriage 152, which in turnis fitted into the larger carriage 150. The lens 54 of the probe 52 isthen positioned over a white, unexposed portion of the processed print130. The control knob 56 is switched to the red channel. Using thecontrol 60, the readout 58 is adjusted to read zero optical density. Thecarriage 150 is then moved to align its aperture 166 with the cyan image136. The assembly of the probe 52 and the smaller carriage 152 is thenmoved laterally along the larger carriage 150 until the readout 58indicates a value of optical density that has been found by priorexperience to provide a satisfactory overall printing density for thetype of color printing paper being used. By way of example, a densityvalue of 0.42 will be used as the balance point in this illustration.When the position has been found that gives the 0.42 density reading onthe cyan image 136, the pointers 170 are set opposite the zero points ofeach of the scales 168. As mentioned above, the scales 168 arecalibrated in opposite directions so that, with the pointers 170 zeroed,a movement of the carriage 152 in the direction of increasing density ofthe image 136 is indicated by a positive displacement on one of thescales 168 with respect to its pointer 170, while the other pointermoves off scale. Similarly, movement of the carriage 152 in thedirection of decreasing density of the image 136 produces a reading ofnegative displacement on the other scale 168 in relation to its pointer170. Using the alternate embodiment discussed above where a single scalewith a center zero point is used, movement of the carriage 152 wouldproduce either positive or negative density readings as the singlepointer is displaced from the zero point.

The steps described immediately above fix the pointers 170 inrelationship to the carriage 150 such that the overall printing densityis keyed to the cyan image 136. The responses of the other twolight-sensitive layers are then related to the response of the cyan dyeemulsion layer as will now be described. The densitometer probe 52 isagain positioned over a white portion of the paper 130, and the control56 is switched to the green channel. The control 60 is then adjusted fora zero optical density reading. The carriage 150 is then positioned withits aperture 166 over the magenta image 134. The assembly of the probe52 in the carriage 152 is moved along the magenta image 134 until thereadout 58 indicates 0.42 optical density units. This completes thecalculation of the green color-balance correction. The correctionfactor, whether positive or negative, will be indicated by the positionof one of the pointers 170 on its corresponding scale 168 making surethat the carriage 150 firmly abuts the rail 146 in the same manner aswhen the pointers 170 were zeroed with respect to the scales 168. Thereading for green color-balance correction is then noted and recorded. Apositive green-correction factor indicates a need for increasing theexposure of green light relative to red light, whereas a negativegreen-correction factor indicates a need for decreasing the exposure ofgreen light relative to red light.

In a similar sequence of steps the blue color-balance correction is thencalculated. In particular, the control knob 56 is switched to the bluechannel and the probe 52 is positioned over white paper, where onceagain the knob 60 is adjusted to give a zero optical density reading.The carriage 150 is then moved to position its aperture 166 over theyellow image 132. The assembly of the probe 52 in the carriage 152 ismoved to find the balance point of 0.42 optical density units. Againmaking sure that the carriage 150 firmly abuts the rail 146, a positiveor negative displacement reading is given by one of the pointers 170 onits scale 168, which is noted and recorded as the blue color-balancecorrection factor, positive indicating a need for increased blueexposure and negative indicating a need for decreased blue exposure.

At this point the color balance of the printing paper from which thetest sample was selected can be objectively related to other lots ofprinting paper previously tested using the same procedural test steps.Very useful quantitative information about the relative responsivenessof the light-sensitive emulsion layers may be obtained, particularly ifthe various tests are always keyed to the same emulsion layer thatserves as the reference layer for the color-balance correction factorsof the other two layers. The information is most reliable when the samereference layer is anchored to the same balance point density in testingseparate lots of printing paper, as for example using the red-lightsensitive layer as the reference layer and always anchoring it to 0.42optical density units in accordance with the above illustration.

It will be appreciated that there is a cooperation between the easel 110of FIG. 4 and the calculator 140 of FIG. 6. That is, the easel 110permits the formation of the three images through the optical wedge 120in a parallel and perfectly aligned fashion so that the displacementmeasurements can be taken from a perpendicular reference line providedby the edge of the rail 146. A variety of alternate arrangements can beconceived for producing similar results. For example, an easel could bearranged with a carriage that pivots about a point such that the threeimages of the optical wedge 120 would appear with the axes of varyingoptical density disposed radially from the pivot point. In such case, acalculator would have to be provided with means for measuringdifferential displacements of points on the images from the equivalentpivot point or from a reference line formed by an arc having theequivalent pivot point as its center. The present invention contemplatesany system for measuring differential displacements of points on theimages that can be referenced to a point or points in the optical wedgeused to make the images, and for the particular purpose of convertingsuch differential displacement measurements to exposure corrections,such that differing exposures of the primary colors of light can begiven to printing paper inverse to the relative sensitivities of thethree emulsion layers to produce a print with the three colors of dye inbalance.

In addition to objective calculation of color balance, the presentinvention facilitates objective measurement of the overall printingspeed of the paper being tested. Accordingly, the control 56 of thedensitometer probe 52 is switched back to the red channel, the lens 54is placed over a white, unexposed portion of the printing paper, and thereadout 58 is zeroed using the control 60. Next, the lens 54 of theprobe 52 is positioned over the uniform cyan image 138, and thedensitometer reading is noted and recorded for later use. This opticaldensity readings serves to relate the responsiveness of the red-lightsensitive layer of the paper under test to the known amount of exposureused to make the uniform cyan image 138. Recalling that the uniform cyanimage 138 was made for a predetermined duration using a predeterminedlight intensity, the intensity being represented by the setting of theneutral channel attenuator 48 of the color analyzer 30, a variety ofmeans can be used to control the overall density of color prints made onpaper of like characteristics to the sample sheet of printing paper 130.For example, if it has been determined that the balance point of 0.42optical density units produces a print of satisfactory overall density,then the differential displacement along the cyan wedge image 136 isfound between a point reading 0.42 and a point having the same readingas the uniform cyan image 138. The differential displacement thusmeasured on the scales 168 provides a printing-speed correction factor,which can be programmed into the color analyzer 30 by adjusting thesetting of the neutral channel attenuator 48 for subsequent printmaking. If the uniform cyan reading is greater than 0.42, then thesetting of the attenuator 48 is adjusted downward by an amount equal tothe printing-speed correction factor. On the other hand, if the uniformcyan reading is less than 0.42, then the setting of the attenuator 48 isadjusted upward by an amount equal to the printing-speed correctionfactor.

The remainder of the specification describes a preferred procedure forusing the color-balance correction factors to print color negatives.Accordingly, the color analyzer 30 is reprogrammed to take into accountthe color-balance correction factors computed above. Though it is notnecessary, it is preferred that the same separation filters and samelamp intensities used in the initial programming of the color analyzer30 are also used in the following reprogramming procedure. It will berecalled that the color balance for green and blue was keyed to thered-light sensitive emulsion layer by zeroing the pointers 170 and thescales 168 with the densitometer probe 52 positioned over the balancepoint on the cyan wedge image 136. Therefore, no adjustment is necessaryfor the red attenuator 42 of the color analyzer 30. The green adjustmentis made by first positioning the probe's aperture 34 in the field ofillumination on the enlarger baseboard 18 with the control 36 switchedto the green channel. Then, working only in the light of the enlarger10, the green adjustment is programmed into the color analyzer 30 in thefollowing manner. The green attenuator 44 is adjusted to move the needle40 on the scale of the meter 38 by an amount corresponding to the amountof optical density units of the green color-balance correction factor.The needle 40 is moved in the direction of increasing optical density ifthe green color-balance correction factor is positive, whereas theneedle 40 is moved in the direction of decreasing optical density if thegreen color-balance correction factor is negative. In the event thatsuch adjustment causes the needle 40 to move off scale, the initialposition of the needle can be reset in a position which will preventthis from occurring preferably by varying the aperture of the lens 16,and then repeating the process of adjusting the setting of theattenuator 44. When the analyzer 30 has been successfully reprogrammedwith the green adjustment, the new setting of the green attenuator 44 isnoted and recorded.

The procedure for reprogramming the color analyzer 30 for the blueadjustment is identical to the procedure just described for the greenadjustment. In particular, the control 36 is switched to the bluechannel, and the blue attenuator 46 is adjusted to deflect the needle 40by either a positive or negative number of optical density unitscorresponding to the blue color-balance correction factor. The settingthus achieved for the blue attenuator 46 is noted and recorded.

Thus the color balance for paper tested in the above described manner isdetermined objectively without resort to visual judgment and withouttying the color balance to any particular color negative. Such colorbalance is represented quantitatively by the settings on the coloranalyzer 30, that is, by the original setting of the red attenuator 42and the new settings of the green and blue attenuators 44 and 46. Theserecorded settings for the color balance of the printing paper may now beadded to color-balance correction factors for the brand and type ofcolor negative film to be printed. There are a variety of knowntechniques for obtaining such correction factors for color negativefilm. In a preferred technique, a standard gray test card isphotographed, and then its negative is analyzed using a color analyzersuch as that shown in FIG. 2. The color analyzer settings for the filmand the paper can then be arithmetically combined to produce finalprinting settings for proper color rendition of the particularcombination of negative film and printing paper used. In order toproduce color photographic prints of superlative color balance, it isonly necessary then to provide a gray card negative for the variousdifferent light settings for the photographed subjects.

Those skilled in the art will appreciate that the above-describedinvention enables the color balance of various lots of color printingpaper to be compared objectively by making only one test print from eachlot. Once the color balance of a box of printing paper is known, anynegative can be printed with a high degree of color excellence, providedonly that the color balance of the film has been determined by anyacceptable technique.

Those skilled in the art will appreciate that the present invention isapplicable to both tricolor additive printing and white-lightsubtractive printing. Once the color-balance correction factors havebeen computed, they can be used to adjust the relative exposures of theprimary colors of light in a variety of different ways. Since there is arelatively wide middle range of response of the emulsion layers of theprinting paper wherein optical density varies linearly with logexposure, the correction factors for the printing paper may be directlyconverted to changes in values of exposure. Within the limits ofreciprocity between time and intensity, exposure variations can beequated to changes in light intensity holding exposure time constant, orchanges in exposure time holding light intensity constant. Moresophisticated computations can also be used to convert the color-balancecorrection factors to various combinations of changes in light intensityand exposure duration. Devices for making such conversions areavailable, such as found in the Kodak Color Dataguide, Kodak PublicationR-19.

It is standard procedure in tricolor additive printing to hold theintensity of the enlarger lamp constant while varying the exposure timesfor each of three exposures of the negative to the printing paper usinga different colored separation filter for each exposure. Therefore,tricolor additive printing may be used in conjunction with correctionfactors calculated in accordance with the present invention byconverting the correction factors to changes in the durations of theexposures. For best results, an electronic timer may be used in thecircuit powering the enlarger's lamp to precisely control exposure time.

Likewise, white-light subtractive printing may be used in conjunctionwith correction factors calculated in accordance with the presentinvention by converting the correction factors to appropriate changes infilter densities of the various subtractive color filters in order toachieve appropriate intensity variations of the three primary colors oflight.

Additionally, in a modified tricolor printing process, instead of usingexposure time variations, it is possible to use intensity variations byemploying the device 26 of FIG. 1 to control the voltage of the enlargerlamp 14 by appropriate amounts for each of the three separate exposures.

Although preferred embodiments have been described in detail, it is tobe understood that various changes, substitutions and alterations can bemade therein without departing from the spirit and scope of theinvention as defined in the appended claims.

What is claimed is:
 1. A method for ascertaining the color balance ofcolor photographic printing paper of the type having superimposedlight-sensitive emulsion layers, comprising the steps:(a) formingseparate test images on a sample sheet of printing paper usingpredetermined linearly varying log exposures of different colors oflight, and (b) measuring the relative responsiveness of the emulsionlayers using differential displacements of like values of opticaldensity on each of the test images.
 2. A method for ascertaining thecolor balance of color photographic printing paper of the type havingthree superimposed light-sensitive emulsion layers, each of which isselectively responsive to a different primary color of light, comprisingthe steps:(a) making three successive exposures of separate portions ofa sample sheet of printing paper through an optical wedge of lineardensity gradient, each exposure consisting essentially of a differentprimary color of light of predetermined intensity and duration, (b)developing the sample sheet of printing paper to produce three separateimages, each image representing the response of a different one of theemulsion layers, and (c) measuring the relative linear displacement ofpoints of equal optical density on each of the three images in unitscorresponding to the linear density gradient of the optical wedge,thereby providing quantitative color balance data.
 3. The method ofclaim 2 further comprising the step of using the displacement data toadjust relative exposures of the primary colors of light in subsequentprinting of color negatives.
 4. The method of claim 2 wherein a fourthuniformly exposed image is formed to permit objective measurement of theprinting speed of the sample sheet of printing paper.
 5. An apparatusfor use with a sample color photographic print and an instrument formeasuring reflection densities at selected points on the sample print,comprising:means for holding the sample print in a fixed position withimages on the print located at a predetermined distance from a referenceline, the holding means comprising a print holder having a flat mountingsurface, and a spring-loaded rail for clamping an edge of the sampleprint against the flat mounting surface, the reference line being formedat the interface of the rail and the sample, and means for measuringrelative displacement of points of known density on the images from thereference line.
 6. The apparatus of claim 5 wherein the displacementmeasuring means comprises a sliding member having an end adapted toengage the rail, the sliding member being equipped with scale meanscalibrated to give optical density readings.
 7. The apparatus of claim 6wherein the displacement measuring means comprises a first carriagehaving an end adapted to slideably engage the rail, a second carriageadapted to slide along the length of the first carriage with a probe ofthe reflection density measuring instrument fixed in an aperturethereof, and means for measuring the relative displacement of thecarriages.
 8. In combination, an easel for forming images of linearlyvarying log exposure to different colors of light on a sample sheet ofcolor photographic printing paper, and a calculator for measuring thedifferential displacement of points of known exposure on the images inunits equatable to differences in exposure of the different colors oflight inverse to the relative sensitivities of the printing paper.